Tracking global variables in Ruby with #trace_var

An not-so-well-known feature of Ruby is #trace_var, which allows to listen to changes in a variable and execute a callback.

$counter = 0

trace_var :$counter do |number|
    puts "the counter is on #{number}"
end

10.times { $counter +=  1 }

#=> the counter is on 1
#=> the counter is on 2
#=> the counter is on 3
# ...

The concept of tracing changes in a variable is interesting - Different objects can listen to these changes, and coordinate their work without stepping in each other's toes. Let's build a simplified Fizz! and Buzz! yellers using #trace_var

class FizzBuzzer
  def initialize(*predecessors)
    trace_var :$counter, ->(number) do
      yell if yelling_condition
    end
  end

end

class Fizz < FizzBuzzer
  def yell
    puts "Fizz!"
  end

  private

  def yelling_condition
    $counter % 3 == 0
  end

end

class Buzz < FizzBuzzer
  def yell
    puts "Buzz!"
  end

  private

  def yelling_condition
    $counter % 5 == 0
  end

end

class NoFizzBuzz < FizzBuzzer
  def yell
    puts "#{$counter}"
  end

  private

  def yelling_condition
    $counter % 3 != 0 && $counter % 5 != 0
  end

end

$counter = 0

fizz = Fizz.new
buzz = Buzz.new
no_fizz_buzz = NoFizzBuzz.new

15.times { $counter += 1 }

While the idea of listening to global variables to drive business logic does not seem sound for production code, you can certainly use #trace_var during debugging to see what could be messing with global variables.

trace_var :$some_global_variable do |value|
  puts "$some_global_variable = #{value}"
  puts caller
end

But if you are certainly in need of tracing variables and set callbacks in depth, Ruby takes this concept further with its TracePoint API.